While voltage-dependent sodium channels in most species share common structural elements, a number of different sodium channel isoforms may be expressed in the same animal or even in the same tissue. The expression of these isoforms is closely regulated, suggesting that each channel isoform has evolved to function optimally under a particular set of circumstances. Mammalian skeletal muscle expresses at least two sodium channel alpha subunit isoforms that are encoded by separate genes; SkMl is the predominanat isoform in adult innervated muscle while SkM2 is expressed in developing muscle and reappears in adult muscle after denervation. These isoforms are independently regulated by electrical activity in culture, and by neuromuscular blockade in mature muscle. The expression of a beta subunit, beta1, is tightly co-regulated with SkM1, but not with SkM2. Although we know at the molecular level the structural elements that make these subunit isoforms unique, little is understood about the mechanisms that control expression of the genes that encode them. The goal of our research program is to analyze the genomic regulatory elements that control muscle isoform gene expression, and to characterize the trans-factors that interact with them. We will determine which aspects of sodium channel gene regulation are shared with genes for other muscle- specific proteins, which are specific to sodium channel genes, and which are unique to a single channel isoform. We will also define how these regulatory elements and trans-factors interact with specific physiological events or conditions. We will work toward this goal by first identifying the regions of genomic DNA that control the expression of the SkM1, SkM2, and beta1 subunit isoform genes in isolated cells in tissue culture, based on the ability of these fragments to drive tissue and developmentally- specific expression of a reporter gene. Candidate promoters and regulatory elements will then be evaluated further for their role in modulating expression of a reporter gene in postnatal development and in adult muscle after denervation, using recombinant adenovirus as a vehicle for introduction of test constructs into muscle in vivo. The role of defined regulatory elements in early development will be validated in vivo by the construction of a limited number of transgenic animals; these animals will also be used to assess positional aspects of expression in adult muscle after denervation. Finally, we will begin to characterize the trans- factors themselves, and to study their interaction with other known factors and signalling pathways in skeletal muscle in vivo.